This disclosure relates to a metal alloy composition that can be used as overlay coatings and/or bond coats in gas turbine engines.
Protection of metal alloy surfaces used in the high temperature regions of gas turbine engines can be affected by the use of overlay coatings, and/or thermal barrier coatings (TBC's) deposited over bond coats. Overlay coatings and TBC's protect the underlying metal alloy substrate against heat and the corrosive environment of the hot gases. Gas turbine components that are typically coated with TBC's and overlay coatings include both moving and stationary parts such as turbine blades and vanes, gas mixing conduits, turbine shrouds, buckets, nozzles, combustion liners and deflectors, and other components subject to the conditions of high heat and corrosive gases. TBC's and overlay coatings typically comprise the external portion or surface of these components. The presence of the TBC and/or overlay coating provides a heat reducing barrier between the hot combustion gases and the metal alloy substrate, and can prevent, mitigate, or reduce potential heat, corrosion, and/or oxidation induced damage to the substrate.
The most effective coatings for protecting metal alloy turbine components are those known as MCrAlY coatings, wherein M is typically cobalt, nickel, iron, or combinations thereof. These coatings are useful as both overlay coatings or bond coats.
The aluminum present in the metal alloy coating can diffuse into the metal alloy substrate, which is undesirable. Such diffusion reduces the aluminum content in the metal alloy composition, aluminum being necessary to allow for the formation of a protective aluminum oxide surface. Cross diffusion of other elements in the surface coating and the substrate, such as nickel, cobalt, or chromium, occurs and is also undesirable.
The metal alloy composition is useful for, among others, bond coats between the TBC and the metal alloy substrate. TBC's are susceptible to delamination and spalling during gas turbine operation. The spalling and delamination can be caused by several factors, including the presence of thermally grown oxide layers (TGO's) that can form at the interface between the TBC and the bond coat interface. TGO formation can be the result of oxidation of the aluminum of the bond coat, and can be promoted by the diffusion of aluminum from the bond coat into the TBC, causing a change in the structure of the bond coat which can further cause a strain mismatch between the TBC and the bond coat. After the TBC spalls, the oxidation of the system is protected by the aluminum content in the bond coat, which forms an aluminum oxide protective layer.
There is therefore a need for metal alloy compositions with improved diffusion properties for use in bond coats. A bond coat with improved diffusion desirably can reduce or delay the onset of spalling and delamination of TBC's.